If you have an Arduino older than Uno Rev3, you may be suffering from the "reset overshoot" problem described here:http://forum.arduino.cc/index.php?topic=64256.105 These were especially common in the Uno R2.The fix is a clamping diode (any general purpose diode) between reset and +5V (cathode on +5V, anode on reset), which you can do like: